Factory automation requires extremely reliable limit switches and position sensors that are immune to electromagnetic induction. Also, certain industries, such as in the petrochemical and energy fields, require intrinsically-safe switch devices in hazardous locations. Further, there is a major effort to develop aircraft and ships with optical fiber control systems, and the automotive industry is also increasing the use of fiber optics.
Various prior art measuring devices are known for measuring physical parameters as a function of light intensity through the use of transducers. In these devices, light from one or more light sources is transmitted via an outgoing optical fiber to an optical transducer which, in proportion to the parameter to be measured, varies the intensity of light transmitted from the transducer to a detector. Typically, a photo-detection apparatus is used to convert the returned light intensity to a suitable electrical signal(s). It is known that mechanical effects (e.g., bending of the fiber, remating connectors, etc.), as well as environmental effects (e.g., temperature, pressure, etc.), can also affect the intensity of the returned light. Multiple light sources and detectors can be used to compensate for these undesirable effects.
As described in U.S. Pat. No. 4,356,396, lights of two different wavelengths are generated. Two detectors monitor the outgoing light, and two detectors monitor the returning light. One of the lights is reflected back by a mirror positioned ahead of an optical transducer, to provide a reference intensity for comparison and compensation calculation. The transducer acts only upon light of the second wavelength. Thus, it is not possible to make corrections for mechanical or environmental perturbations in the transducer and transmission components.
Optical transducers acting on lights of more than one wavelength are also known. For example, U.S. Pat. Nos. 4,514,860, 4,417,140 and 4,378,496 disclose devices having two light-emitting diodes and associated electronic components in the transducer assembly. A transducer operating without any electric current or voltage is desired, however, to reduce the hazards associated with electrical devices, especially in harsh environments.
Further, multiple wavelengths of light have also been used in prior art transducer assemblies to make multiple measurements. This known technique, as described in U.S. Pat. Nos. 4,523,092, 4,493,995 and 4,281,245, does not compensate for environmental or mechanical perturbations in the transducer. Another example of using two wavelengths is described in U.S. Pat. No. 4,492,860, but only a single fiber is used and polarization is employed to control reflection of the outgoing light into detector apparatus.
Thus, upon information and belief, it has been necessary in the prior art to regularly and carefully inspect the integrity of the optical components to ensure the relevant measurements are not "false" readings, or readings significantly affected by stray light or optical failure. As set forth in detail herebelow, however, one of the primary objects of the present invention is to provide self-monitoring means for determining the optical continuity and light-sealed integrity of the optical path, thereby rendering regular inspection and special testing of the optical components unnecessary.